As shown in Patent Document 1, for example, a conventional distortion compensating apparatus has a problem in that its distortion compensation performance deteriorates because of the gain error and phase shift error of the π/2 phase shifter between the in-phase (I) channel component and quadrature (Q) channel component in a quadrature modulation circuit or quadrature demodulation circuit. Here, the gain error and the phase shift error are collectively called IQ imbalance.
In contrast with this, the Patent Document 1 circuit, in addition to the conventional distortion compensation circuit, compensates for the IQ imbalance by causing a waveform generating circuit to output a predetermined baseband signal, and by installing an error measuring circuit for measuring a demodulation error by extracting a demodulation signal corresponding to the baseband signal, and an error correcting circuit for eliminating the demodulation error from the demodulation signal.
In addition, although not describing the distortion compensating apparatus, Patent Document 2 and Non-Patent Document 1 each describes a technique for compensating for the IQ imbalance involved in the quadrature modulation.
The Patent Document 2 circuit compensates for the IQ imbalance by linearly converting the in-phase amplitude signal and quadrature amplitude signal with a linear converting means; by extracting a level signal by supplying a level signal generating means with a modulation wave output of the quadrature modulator to which the linearly converted signal is input; and by causing a parameter generating means to derive linear converting parameters for the level signal to be used for the linear conversion.
Although the first embodiment of the Patent Document 2 obtains the linear converting parameters from the level signal using a known test signal, the second embodiment obtains the linear converting parameters from the level signal in accordance with an in-phase and quadrature amplitude input. More specifically, instead of using the special test signal, the second embodiment calculates the linear converting parameters for the compensation by using particular values of a transmission signal such as values at the moment when the I component or Q component becomes zero, or when the I component becomes equal to the Q component.
Furthermore, the Non-Patent Document 1 describes an idea of adjusting the IQ imbalance of a quadrature modulation circuit and a quadrature demodulation circuit by using a loopback signal. However, it does not describe a concrete error detecting method or adjusting method.
Patent Document 1: Japanese patent application laid-open No. 6-268703/1994 (Paragraphs 0014 and 0019).
Patent Document 2: Japanese patent No. 3144649 (Paragraphs 0026-0028 and 0065-0068).
Non-Patent Document 1: Kohri, Yamamoto, Kawanaka, “Study of Automatic Adjusting Circuit Using TDMA-TDD” Proceedings of the 1993 IEICE Spring Conference, Mar. 15, 1993, No. 2, B-314.
It is necessary for the conventional distortion compensating apparatus using the technique of the Patent Document 1 or of the first embodiment of the Patent Document 2 to transmit the particular test signal to determine the IQ imbalance compensation coefficients (the linear converting parameters of the Patent Document 2) for compensating for the IQ imbalance. However, mobile communication or broadcasting base station equipment to which the distortion compensating apparatus is applied cannot transmit the particular test signal after their operation has been started because they operate continuously all day long. Thus, the IQ imbalance is compensated for by obtaining the IQ imbalance compensation coefficients only once by using the test signal before starting the operation after installing the base station. Accordingly, the distortion compensating apparatus has a problem of being unable to cope with temperature changes or aging phenomena after starting the operation, thereby deteriorating the distortion compensation performance.
In addition, it is necessary for the distortion compensating apparatus utilizing the technique of the second embodiment of the conventional Patent Document 2 to use the particular values of the transmission signal to determine the IQ imbalance compensation coefficient. However, there is no guarantee that the transmission signal with the specified particular values appear during the actual operation, and even if it appears it takes a long time before the appearance because the specified values are very rare and the probability of appearing is low. Thus, the distortion compensating apparatus has a problem of taking a long time before compensating for the IQ imbalance.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a distortion compensating apparatus capable of obtaining the IQ imbalance compensation coefficients in real time using any given transmission signal, and capable of preventing the deterioration in the distortion compensation performance in spite of the temperature changes or aging after starting the operation.